Voltage levels in distribution networks for electrical energy depends on several factors, such as the location within the distribution network, e.g., a certain location along the extension of a distribution or transmission line, or the actual load condition. The voltage level is subject to continuous variation therein.
Medium voltage, in the context of this application, refers to a typical voltage level of a distribution network between roughly 50V and 36 kV. In order to keep the voltage level for the loads of a distribution network within a desired range around the nominal voltage of a respective part of the distribution network, e.g. a nominal voltage level of 10 kV, it is known to provide tap changers for the distribution transformers of the distribution network.
Tap changers have the ability to adapt the voltage ratio of a transformer, for example with a rated voltage of 36 kV on the primary side and 6 kV on the secondary side and with a rated power of 1 MVA, within a certain range, so that the voltage on the output side of the transformer is within a desired range around the nominal voltage. Tap changing of transformers requires a high effort for construction since additional taps for the transformer winding have to be foreseen as well as an additional tap changer which is a selector switch between the different taps. Due to the high current which might have to be switched by the tap changer during load condition tap changers are subject to an increased wear.
It is also known that, especially for long transmission lines, e.g. with lengths of 10 km or more, a continuous voltage drop or even a voltage rise along the transmission line might occur dependent on the actual load conditions. In this case it might not be possible to provide a voltage level for each location along the whole length of a transmission line that is within an admitted tolerance level around the nominal voltage. At one end of the transmission line the voltage level might be below the admitted tolerance range even on the other end the voltage level is at the upper edge of the admitted tolerance range.